1. Field of the Invention
This invention is related to devices for holding milling cutters (tools) in mills and particularly to a toolholder that enables a user to make a quick and convenient substitution of tools without removing the collets.
2. Background and Information Disclosure
A mill for cutting a metal part including a vise to hold the part and means to move the vise and part in three dimensions while machining (cutting) the part with a rotating tool has been in widespread use for almost a century. One particular manufacturer has been so successful in marketing a version of this machine that the machine is universally referred to as the "Bridgeport mill" although there are numerous other manufacturers all producing an identical machine.
The toolholder used in the Bridgeport mill is a collet with a bore having a thread on one end. The collet slips into the bottom end of the bore of a rotatable spindle. A drawbar inserted into the top of the spindle is screwed into the end of the collet and draws the collet into the bore of the spindle. The shank of a tool is positioned in the bottom end of the collet. The collet has three slits along the shank of the tool so that as the low end of the drawbar is screwed into the top end of the collet thereby drawing the collet into the bore of the spindle, the lower end of the collet is squeezed in the flared end of the bore of the spindle so as to clamp onto the tool. This construction provides very solid support to the shank of the tool This is an important requirement in order to avoid vibration which would occur as the tool cuts the metal and cause degradation of the finish of the surface of the part. The fluted construction of the collet with slots also maintains very accurate concentricity of the shank of the tool with the rotational axis of spindle. This is necessary for achieving good finish and precision and is the arrangement that has been used by millions of machinists over a period of fifty years.
A major inconvenience with this arrangement is that the machinist must change his toolbits frequently, inserting larger or smaller tools or he may be required to insert a drill bit requiring that an expandable chuck with a straight shank be mounted in the spindle. Changing one tool to another tool having a different shank diameter also requires that he must change one collet to another collet having an appropriate bore to fit the diameter of the shank. This involves the time consuming operation of separating (by unscrewing) the drawbar from the top end of the collet, removing the collet from the bore of the spindle, searching for the collet having the appropriate bore diameter to fit the shank of another tool, inserting the second collet back into the bore of the spindle, screwing the drawbar back onto the end of the newly mounted collet. Because the collet universally used for this operation is typically eight inches long, the machinist must usually operate the crank handles of the mill to move the vise and part down and away from the end of the spindle in order to remove one collet and insert another collet. This causes loss of registration of the part with the tool bit and consequently loss of accuracy in machining the part. Typically, the requirement to change collets in machining a pan extends the time required to machine the part by about 50%.
Another problem with this arrangement is that the machinist must have available a number of expensive collets, each collet having a bore diameter different from the bore diameters of the other collets such that there is one collet for every shank size of the machinist's collection of toolbits. This requires a considerable cost to the machinists because each collet is precision machined and has a taper on an outside surface, a thread in one end, a precision bore, axial slots that allow the collet to squeeze onto the end of the shank, and the holder must be machined from hardened steel in order to extend the life of the threads formed in the bore.
Because of the expense of the collet, the typical machinist generally has only one collet per shank diameter and therefore has a limited number of collets. Therefore, he typically maintains his tools in a place separate from where he stores his collection of collets. The result is further loss of time in sorting through his collection of collets and collection of cutters in order to match the collet with the cutter.